The present invention relates to the electronics arts. It is especially relates to group III-nitride flip-chip bonded light emitting diodes for lighting applications, and will be described with particular reference thereto. However, the invention also finds application in conjunction with other types of flip-chip bonded light emitting diodes, and in other flip-chip bonded epitaxial semiconductor devices such as vertical cavity surface emitting laser diodes.
In the flip-chip mounting configuration, a light emitting diode with a light-transmissive substrate and front-side electrodes is bonded “face down” to bonding bumps of a mount, that is, with the epitaxial layers proximate to the mount and the light-transmissive substrate distal from the mount. The flip-chip arrangement has a number of advantages, including improved thermal heat sinking due to the proximity of the front-side active layers to the heat sinking substrate, and reduction of electrode shadowing losses.
In the flip-chip mounting configuration, light is extracted from the substrate side. For epitaxially grown light emitting diodes, the choices for substrate material can be highly restricted since the substrate is selected principally to provide a good base for the epitaxy. Thus, the substrate criteria include a narrow lattice constant range, a substantially atomically flat surface for nucleation of epitaxy, thermal stability at epitaxial growth temperatures, chemical compatibility with the epitaxial process, and so forth.
A problem can arise in the flip-chip configuration when the growth substrate is substantially light-absorbing over some or all of the spectral range of light emission. In this case, light extraction from the substrate is reduced due to light absorption losses in the substrate. Moreover, even if a suitable optically transparent substrate is available, such as is the case for group III-nitride light emitting diodes which can be grown on a transparent sapphire growth substrate, reflection optical losses can occur at the interface between the substrate and the epitaxial layers due to an abrupt discontinuity in refractive index.
A known approach for reducing these substrate-related optical losses is to transfer the epitaxial layers stack from the light-absorbing growth substrate wafer to an optically transparent wafer. Typically, this involves intimately bonding the epitaxial layers stack to the optically transparent wafer, and then removing the growth substrate wafer by etching. After removal of the growth substrate, the epitaxial layers stack remains bonded to the transparent wafer, which is then processed to fabricate devices, and diced to separate individual light emitting diode die. However, achieving intimate bonding between the epitaxial layers stack and the transparent substrate over large areas is difficult. Device yield can be compromised due to the formation of air bubbles or the presence of particles at the interface between the epitaxial layers stack and the transparent substrate during the bonding. Moreover, absent a close refractive index match between the epitaxial layers stack and the transparent substrate, reflections at the interface between the layers stack and the transparent wafer can introduce optical losses.
Another approach is to temporarily secure the epitaxial layers stack to a temporary support wafer using an adhesive layer, followed by thinning of the growth substrate. The epitaxial layers stack, with the remaining thinned growth substrate adhering thereto, is then detached from the temporary support wafer and processed and diced to produce light emitting diode die. The light emitting diode die, which have thinned substrates, are flip chip bonded to a mount. However, the epitaxial layers stack and the remaining thinned growth substrate form a fragile structure after growth substrate thinning. The fragility of this thinned structure complicates the further processing, dicing, and flip chip bonding, resulting in lowered device yield. Moreover, air bubbles, particles, or other imperfections in the adhesion between the temporary support wafer and the epitaxial layers stack can introduce localized damage to the thinned structure, also impacting device yield.
The present invention contemplates an improved apparatus and method that overcomes the above-mentioned limitations and others.